Electric control system



1946' G. M. ADAMS ET AL ELECTRIC CONTROL SYSTEM Filed Oct. 8, 1943 2 Sheets-Sheet 1 Inventors. George M. Adams, John C. Aydelott, b5

T eir' Attorney.

Jan. 29, 1946. ADAMS ET AL 2,393,620

ELECTRIC; CONTROL SYSTEM F' iled Oct. 8, 1943 2 Sheets-Sheet 2 88f 07 Fig. 5.

m 0 b: c A /3/ l l L 5 IEl l r I I 5 l 1 v AMPS I000 2000 LINE C l/HHE N7 VOL M65 Fig. 4.

HANDLE TRAVIL Inventors:

@ George M. Adams,

John C. Agdelott, 6% 78 by I Their Attorney.

Patented Jan. 29 1946 ELECTRIC CONTROL SYSTEM George M. Adams and John C. Aydelott, Erie, Pa., assignors to General Electric Company, a corporation of New York Application October 8, 1943, Serial No. 505,468

Claim (Cl.17 179) l Our invention relates to electric control systems, and is particularly applicable to gasor Diesel-electric power systems for self-propelled vehicles.

The control system forming the subject matter of this application is an improvement upon the system disclosed and claimed in a copending application, Serial No. 469,538, filed on December 19, 1942, as a joint application of Martin A.

Edwards, Donald E. Garr, John C. Aydelott, and

George M. Adams, and also in the copending applications, Serial No. 505,437 and Serial No. 505,469, filed concurrently herewith by Martin A. Edwards and George M. Adams, respectively, and assigned to the same assignee as the instant application. The aforementioned joint application is being amended as a joint application of Martin A. Edwards, Donald E. Garr, and John C. Aydelott. The inventions of said joint application and said Edwards application were made prior to our invention and I do not claim herein anything shown or disclosed in said applications,

which are to be regarded as prior art with respect to our present invention.

' Diesel-electric power systems for self-propelled vehicles ordinarily comprise a prime mover, such as a Diesel or other internal combustion engine, arranged to drive a generator from which power is supplied to one or more traction motors. The systems of this type which. are in more or less common use at the present time make use of traction motors of the series field type and rely upon voltage control of the generator for controlling the torque and speed of the motors, and reversing switches for controlling their direction of rotation. While such a system is suitable for application to vehicles in which all motors simultaneously undergo like changes of speed and torque, such as in Diesel-electric locomotives, buses, and the like, they are not sufficiently flexible to supply the demands of certain other types of vehicles in which an optimum maneuverability is desired. This is especially true of such vehicles as tractors, electric shovels, military tanks, and the like, which rely upon the independent con-, trol of laterally spaced driving elements for propulsion, steering, and braking. Essentially the same problems of propulsion, steering, and braking are encountered in twin-screw or paddlewheel boats, although the difliculty in these applications is minimized to some extent insofar as extreme rapidity of response, while it may be provided, is ordinarily not necessary.

For applications of the above nature, controlled current systems have been proposed. By such a system we mean one in which the main generator is connected in a loop circuit with the motor armatures and arranged to supply to the arma-t tures a constant or otherwise continuously con{ trolled unidirectional current, while theexcitation of the motor field windings is separately and independently controlled to determine the speed, torque, and direction of rotation of each of the motors. 'Such a system is shown in broad outline in British Patent 226,960, and satisfactorily meets the principal demands of maneuverability outlined above. For example, it will be evident that both motors may be suddenly reversed to eflect dynamic braking merely by reversing the relatively small energizing current to their fileld windings, while the unidirectional armature current is maintained constant or within predetermined limits. Similarlythe torque of any one motor may be reduced or, in fact, reversed to elfect steering. It is also evident that an inherent advantage of this arrangement is that it during steering, one of the motors is exerting a reverse torque, or in other words, is being braked and acting as a generator, the power output of the generating motor is supplied to the motor or motors exerting a. forward motoring torque thereby minimizing the amount of power necessary to be supplied by the main generator. The power saving resulting from such an arrangement, as compared with one in which the braking effort is dissipated in heat, has been shown to be substantial over a course consisting of a large number of turns, such as may characterize the run of a military tank, or the like.

While controlled current systems of the general type shown in the British patent above referred to are suitable and satisfactory for marine applications, they have not heretofore been ap-'- plied to land vehicles orv to industrial applications in spite of their obvious advantages with respect to maneuverability. The reason is evi dentwhen it is appreciated that, while the system of the British patent meets the principal demands of maneuverability, it is not characterized by rapid speed of response. Such response is not ordinarily necessary in ship propulsion apparatus. A land vehicle on the other hand, especially one which is to be operated in close a proximity to a number of other vehicles and over irregular terrain, must possess notonly maneuverability but also an extremely rapid rate of response. Furthermore, certain characteristic problems of land vehicle operation, such as overspeeding in running downhill, must be provided against by suitable protective means. Similar problems also are encountered in certain industrial processes where the loan at times overhauls the motor.

Accordingly, it is a general object of our invention to provide an improved electric control system, of substantially unlimited flexibility which is reliable, easy to operate and possessed of a very rapid rate of response.

It is another object of our invention to provide a new and improved traction motor control system having a continuous transition from accelerating to electric braking operation and arranged for prompt and simple transfer from one phase of operation to the other at any time without danger of overloading or overspeeding of any of the elements of the. system,

It is still another object of our invention to provide an electric traction motor control system for a plurality of motors so arranged that any or all motors may be independently trans- !erred between accelerating to electric braking operation substantially instantaeously and in a continuous manner without overloading or overspeeding of any part of the system.

It is another object of our invention to provide in a gasor Diesel-electric controlled current system means for providing a maximum current limit and means for changing the current limit value in accordance with steering demands of the vehicle.

It is a still further object of our invention toprovide, in an electric motor control system able maximum value and means for controlling the current limit value in accordance with the degree of steering demanded.

Our invention alsoihas for its object the provision, in a, controlled current electric driving system for a track-laying vehicle, of means for temporarily raising the maximum current limit to permit abrupt steering operation.

According to our invention the above and other objects are attained by providing an electric power system, and particularly, a gasor Dieselelectric power system comprising a prime mover, a generator, and a plurality of traction motors in which a unidirectional direct current of continuously controlled and definitely limited magnitude is circulated through the motor armatures, and means are provided for separately controlling the excitation of the motor field windings independently to determine the speeds,

torques, and directions of rotation of the motors. By proper selection of forward and reverse torques for the separate motors, steering and forward or reverse motoring or electric braking operation may be effected without affecting the direct control of the continuously circulating armature current. To facilitate steering, means are also provided for automatically increasing the torque of the motors on one side of the vehicle whenever the torque of the motors on the other side of the vehicle is decreased. Severe steering is further facilitated by providing automatic means for temporarily raising the maximum current limit when the motors on only one side are reversed, while forward torque is retained on the motors on the other side of the vehicle.

' Preferably we provide an engine driven generator having an output circuit including the arma tures of all the traction motors connected in series circuit relation. The generator may be of the constant current type, although for more efficient utilization of the maximum available engine horsepower over a wide range of vehicle speed, it is preferable to limit the voltage-current characteristic by maximum available engine horsepower in the region between maximum generator voltage, as determined by saturation, and maximum generator current, as determined by armature reaction or a suitable current limit circuit. Each traction motor is also provided with a separate engine driven exciter provided with manually controlled excitation means to permit a smooth or continuous transition of motor excitation from maximum forward excitation to maximum reverse excitation. For any predetermined setting of the manually adjustable motor field control means, the motor exciter field and hence the motor field energization may be constant or may be connected to vary in accordance with motor current, motor voltage, or motor speed, or any combination of these. In this system line current is limited by an electric valve spillover circuit for reducing the generator excitation upon occurrence of a predetermined current, the spill-over value being reduced as vehicle speed increases, and increased to meet certain demands of steering. An electric valve spillover circuit is also arranged to reduce the power output of the motors whenever the vehicle attains a predetermined limiting speed.

For a more complete understanding of our invention and a further appreciation of its objects and advantages, reference should now be had to the following detailed specifications taken in con junction with the accompanying drawings in which Fig. 1 is a schematic circuit diagram of connections for an electric power system embodying our invention; Fig. 2 is a simplified circuit diagram of the current limit circuit: Fig. 3 is a simplified schematic circuit diagram 0! the vehicle speed limit circuit; Fig. 4 is a simplified schematic circuit diagram of the main motor field energizing circuit; and Figs. 5, 6, '7 and 8 are graphical representations of various operating characteristics of the system.

Referring now to the drawings, and particularly to Fig. l, we have illustrated schematically therein a Dieselor other gas-electric power system for a self-propelled vehicle. The system comprises a prime mover IU of the internal combustion engine type, such as 8. Diesel engine, arranged to drive through a common shaft II, a main generator I2. a generator exciter l3, and a plurality of traction motor exciters I4 and It. An output circuit from the armature o! the generator I2 is completed through the armatures of a pair of traction motors Ii'and H. The output circuit of the generator I! is a series or loop circuit and includes a commutatlng pole winding 18 for the generator I! and commutating pole windings l9 and 20 for the motors Ii and I1, respectively. The generator output circuit may be permanently closed, as shown, or if desired, suitable disconnecting switches may be included. We have also shown permanently connected in the output circuit a braking resistor 2| arranged to be shunted during motoring operation by the contact 22 of a braking contactor B. It will be understood, of course, that the representation of a pair of traction motors I6 and I1 is purely diagrammatic, and that each motor shown on the drawings, may represent either a single motor or a group of motors arranged either in series or parallel circuit relation. For purposes of illustration it may be asvings, an engine throttle lever 23 is controlled through an overtravel linkage 24 from a goveernor 25 to maintain constant the speed of the engine Hi, the setting of the governor being manually controllable by means ofa foot pedal 25 through a speed setting linkage 21 'to select "the desired constant speed. By way of example,

we have shown a fiyball governor 25 arranged to rotate a governor shaft 28 through a lever 29.

The governor is biased toa normal collapsed position by a tension spring 29. To the shaft 28 is rigidly connected an arm 30, and spring biased into following engagement with the arm 30 is a U-shaped link 30' which is loosely mounted upon the shaft 23 and rigidly connected to the throttle lever 23. This mechanisrnis so arranged that upon a decrease in engine speed the inward movement of the fiyballs will. rotate the shaft 28 in a clockwise direction as shown on the drawings, thereby to open the throttle and bring the engine speed substantially back to normal. The link 30 is also provided with a projection 3| arranged to engage a fixed stop 32 in the full open position of the throttle, thereby to prevent further movement of the throttle 23 but to permit overtravel of the lever 30 for a purpose to be more fully described hereinafter.

The speed setting of the governor 25 is controlled by a tension spring 33 connected between the link 29 and a bell-crank lever 34 in aiding relation with the spring 29'. The bell-crank lever 34 is connected through a suitable linkage,

such as a hydraulic system 35, to the foot pedal 28 and is so arranged that depression of the foot pedal applies tension to the spring 33. We have also shown connected to the speed setting bellr crank lever 34 through a lost-motion linkage 3.6

a throttle solenoid 31. As will be explained more fully hereinafter, the throttle solenoid 31 is arranged to apply to the speed setting spring 33 an initial tension as soon as excitation is applied to the field winding of the generator l2, thereby to raise the engine speed to a certain predetermined minimum above idling speed. The lost motion of the link 38 permits further application of tension to the speed setting spring 33 through the foot pedal 26.

Generator field excitation The main engine driven generator I2 is provided with a main field exciting winding 38 connected directly to the terminals of the dynamo electric excite'r l3 and so controlled as to provide in the output circuit of the generator i2 a continuing unidirectional flow of direct current ha ing a predetermined maximum or limiting value regardless of the back electromotive force of the traction motors i5 and H. The volt-ampere characteristic of the generator l2 at any predetermined engine speed is dependent upon the energization of the main generator field winding 38 which is in turn controlled by the energize-- tion of a plurality of control field windings 39, 40 and 4| on the exciter l3.

Preferably the exciter I3 is a direct current generator of the type described and claimed in Patent 2,227,992, issued to Ernst F. W. Alexanderson and Martin A. Edwards on January 7, 1941. Such a generator is of the armature reaction excited type and is provided with a pair of current supply brushes 42, a series load compensating winding 42a, and a pair of short circuited brushes 43. The control field windings 38, 40, and 4| are arranged to generate a voltage between the short circuited brushes 43, and the armature reaction of the current flowing through the short circuited connection as a result of this voltage sets up a fiux in such a direction as to provide an output voltage at the load ,brushes 42 which is proportional to the excitation of the machine along its short circuit axis. One of the desirable characteristics of an armature reaction excited generator of this type is that its output voltage responds very rapidly and with a high degree of amplification to any change in the energization of its control field windings. For this reason machines of this type are frequently referred to as dynamoelectric amplifiers.

Referring now-more particularly to the control field windings of the generator exciter II, the winding 38 is a stabilizing or antihunt winding connected directly across the load terminals of the exciter in series with a capacitor 44; The field winding 39 is thus excited only upon a change in exciter voltage and in a direction to oppose the change. The main control of the generator exciter voltage is attained through the generator exciter forward field 4| and the differential field 40. Fundamentally, the forward control field winding 4| is excited from a substantially constant voltage source of control power, such as a battery 45, while the differential across the terminals of the battery 45 through a circuit which may be followed from the positive terminal of the battery through a. disconnect switch 41, a suitable protective fuse 43, a wire 13+, a normally open interlock contact 43 on the braking contactor B, a resistor, the potentiometer 45, a resistor 5|, and a ground wire 52 which in operation is connected to the negative terminal of the battery 45 through ground and one or the other of a pair of manually operable dead man interlock contacts 53, 54 which will be more fully described hereinafter, The control field winding 4| itself is connected across the potentiometer 46 through a pedal controlled varisprings 29 and 33 normally hold the throttle and overtravel linkages in the positions shown. When the engine is set in operation the governor balls immediately fly out and close the throttle down to its idling position as determined by the initial tension of the springs 29 and 33. In moving the throttle to its idling position the governor also moves the slider of the potentiometer 46 to its extreme.counterclockwise position as viewed at Fig. 1. It will'now be observed that the potentiometer 46 is arranged to effect no change in the voltage applied to the control field winding 4i during movement of the throttle from its idling position to its full open position. If, however, when the throttle is in its full open position, the engine'is still unable to maintain the desired speed, the governor 25, through the overtravel 1inkage'24, will continue to move the lever 30 and the slider of the potentiometer 46, even though no further movement of the link 30 and the throttle lever 23 is possible because of the stop 32. Such overtravel of the governor will, through'the potentiometer 46, reduce the voltage applied tdthe generator exciter forward field winding ll, thereby to reduce the excitation of the generator exciter lii and the main generator 12 and to permit the engine to again come up to the desired speed. By this arrangement the power demand of the engine driven generator I2 is reduced in the middle range of its volt-ampere characteristic where it normally exceeds the maximum available horsepower output of the engine [0, thereby to prevent slight engine stalling within this range and to permit maximum utilization of all available engine horsepower over a wide range of vehicle speed.

The purpose of the pedal rheostat 55 is to select a suitable generator excitation in accordance with the selected engine speed to permit easy starting of the vehicle and easy maneuvering at low speed. The rheostat 55 is arranged to effect no change in the speed range but to reduce the generator exciter field resistance over an initial portion of the field resistance over the major portion of the throttle travel. Thus as the slider of the throttle resistor 55 is moved from its extreme counterclockwise idling position by accelerating movement of th foot pedal 26, the resistance 55 is gradually excluded from the circuit and the exciter excitation built up as the engine speed increases. Preferably, the resistor 55 is arranged to be completely out out at a relatively low engine speed. For example, if the engine is arranged to have a minimum running speed of 1200 R. P. M. with the throttle solenoid 31 closed, it has been found suitable to completely cut out the resistor 55 when the engine reaches a selected low speed of approximately 1400 R. P. M., as has been indicated on the drawings. Thus it will be observed that for throttle openings greater than that corresponding to the selected low speed and less than full open neither the governor potentiometer 46 nor the rheostat 55 exerts any control of the energization of the control field winding 4! so that energization of the winding 4! is substantially constant within the range of throttle positions. It will be understood that variation of the field resistance 55 is not fundamental but that, if desired, a fixed resistor may be substituted therefor. Beyond the full open position of the throttle the governor 25 functions to reduce the power demand of the generator to equal the maximum available horsepower output of the engine by control of the generator forward field 4! through the potentiometer 46.

As previously mentioned, the control field winding is a differential winding and is arranged when energized to excite the generator exciter field in opposition to the flux of the winding II. The field winding 40 is connected across the commutating pole windings l8, l9, and 20 in the generator output circuit and is thus energized in accordance with main line current, thereby to cause the output voltage of the generator I: to drop from a predetermined maximum value to a value just sufficient to supply the resistance drop of the output circuit when maximum current is flowing. The normal energization circuit of the winding 40 may be followed from the positive terminal of the generator l2 through the winding 40 and a current limiting resistor 51 to ground.

Line current limit For the purpose of limiting the current in the generator output circuit to a predetermined maximum value, supplementary means are provided for abruptly increasing the energization of the exciter differential winding 40 when the line current attains such predetermined maximum value. The current limit circuit may be more readily understood from the simplified diagram of Fig. 2. At Fig. 2 the connection of the exciter differential winding 40 across the commutating pole windings I8, I9, and 20 in series with the resistor 51 is clearly shown. To provide maximum current limit, the resistor 51 is shunted by a circuit including the battery 45, a rectifier bridge 58, and a normally open interlock contact 59 of the braking contactor B. As pointed out hereinbefore, the braking contactor B is picked up during motoring operation so that the shunt circuit around the resistor 51 will be closed. It will be observed that both arms of the rectifier bridge are connected to block any flow of current from the battery through the resistor 51. Furthermore, the generator polarity is such that when current is flowing through the output circuit in the normal direction, the side of the resistor 51 connected to the negative side of the battery is negative, and the side of the resistor connected through the rectifier bridge to the positive side of the battery is positive, so that the voltage drop across the resistor 51 opposes the v voltage of the battery through the blocking rectiher bridge 58, the rectifiers preventing any flow of current through the loop circuit from the battery. It will now be evident that when the current flowing in the output circuit of the generator l2, and hence the current flowing through the exciter differential winding 40 and the resistor 51, attains such a value that the voltage drop across the resistor is greater than the voltage of the battery 45 by an amount sufficient to break down the rectifiers in the arms of the bridge 58, current will spill over from the positive terminal of the resistor 51 through the battery 45 thereby to provide'a current path parallel to, the resistor 51 and in series with the differential control field 40. By thus introducing a shunt connection around the resistor 51 the net resistance of the circuit through the control field winding 4|] is reduced and the energization of the winding 40 is sharply increased thereby to diminsh the excitation of the generator exciter l3 and cut down the excitation of the main generator l2. The main line current at which the above spill-over operation takes place is the maximum current limit for the generator output circuit. The added energization thus supplied to the control field winding 40 is sufficiently great so that no further increase in the main line current is possible after operation of the current limit circuit.

Viewed in another way, the above current limit circuit may be regarded as limiting the potential of the positive terminal of the generator l2 to the battery potential plus the small resistance drop through the control field 48. Thus, whenever the positive terminal of the resistor 51 exceeds battery potential suiliciently to render the rectifier bridge conducting, the positive terminal of the battery 45 is efiectively tied directly to the positive terminal of the generator l2, since the rectifiers in the bridge 58 and the winding are of low resistance. Any tendency of the line current to raise the potential of the positive generator terminal above battery potential results in a large flow of current through the control field winding 48 sufficient to reduce the generator excitation to a value only great enough .to maintain the limited current.

It is desirable to reduce the maximum current limit as the vehicle speed increases, that is, the current limit circuit should be brought into operation at a somewhat lower line current when the vehicle speed is high than when the vehicle speed is low. This condition is imposed by the limits of good commutation by the traction motors I 6 and I]. It is well understood that the maximum current which may be satisfactorily commutated at high speed is less than that which may be satisfactorily commutated by the same machine at a lower speed. For this purpose we provide a tachometer generator 60 having a separately excited field winding 6i energized directly from the battery through the wire B-+-. The tachometer generator 60 is driven from the shaft of one of the traction motors, for example, the traction motor l6, and thus supplies an output voltage proportional to vehicle speed for modifying the operation of the current limit circuit. From the discussion of the preceding paragraph it will be recalled that the shunt circuit across the resistor 81 is connected through diagonally opposite points of the rectifier bridge 58. These points have been identified as 62 and 63 in Figs. 1 and 2. Referring now particularly to Fig. 2, it will be observed that a potentiometer 64 having voltage taps 65 and 85a is connected across the output terminals of the tachometer generator 88 to supply to the other the rectifier bridge 58 a suitable voltage proportional to vehicle speed.

The speed voltage between the terminals 66 and 61 of the rectifier bridge 58 is normally derived between the positive terminal of the potentlometer 64 and the tap 65. As shown at Fig. 2, the rectifier bridge terminal 66 is connected to the positive terminal of the potentiometer 64. Between the taps 65 and 66a of the potentiometer 88 are connected a pair of potentiometers 85 and 86 in parallel circuit relation. The potentiometers 86 and 88 are effective only in steering operation in a manner hereinafter to be described. A pair of switches 81 and 88 serve selectably to connect the bridge terminal 61 either directly to the potentiometer tap 68 or to the slider of one of the potentiometers 85, 86. The switches 81 and 88 and the potentiometers 85 and 86 serve temporarily to' raise the current limit for severe steering operation. This modification of the current limit by steering will be more readily understood after the steering operation itself has been described. For the present purpose or understanding the lowering of current limit as vehicle speed increases it is sufilcient to neglect the potentiometers 85 and 86, since these pair of diagonally opposite points 68 and 61 of potentiometers are inefi'ective in straight running and braking operation. Accordingly, unless steering is demanded, the bridge terminal 61 may be regarded as connected directly to the potentiometer tap 65.

Due to the rectifying action of the bridge 58 the speed voltage across the terminals 66 and 61 may be regarded as appearing permanently between the points 63 and 62 of the bridge regard less of the direction of motion of the vehicle, the point 63 being positive with respect to the point 62. From Fig. 2 it may also be observed that in the loop circuit comprising the battery 46, the rectifierbridge 58, and the resistor 81, the voltage drop across the resistor 51 and the speed voltage across the points 62 and 63 of the rectifier bridge 58 are connected additively and in opposing relation to the voltage of the battery 46. Thus for current limit operation the total voltage available for opposing the battery voltage to effect a spillover of current through the rectifier bridge and the battery from the positive terminal of the resistor 51 is increased in proportion to the speed of the vehicle by the voltage between the points 62 and 63 of the rectifier bridge. Since the battery voltage is constant, it will be clear that as the vehicle speed increases, spill-over through the battery and hence current limit operation will take place with progressively smaller voltage drops across the resistor 61, that is, at reduced main line current, since the voltage drop across the resistor 51 is proportional to the main line current. In this way the maximum current limit is reduced as the vehicle speed increases.

Traction motor field control Referring now to the field control for the traction motors l6 and II, it may first be noted that each motor is provided with a main separately excited field winding, the motor l8 having a winding 18 connected to the output terminals of the dynamoelectric exciter l4 and the motor I! having a field winding II connected to the output terminals of the dynamoelectric exciter l6. Preferably the exciters I4 and I 6 are machines of the armature reaction excited type, as heretofore explained in connection with the generator exciter l3 and include series load compensating windings 10a and II a, respectively. The exciters I4 and I5 are provided also with main control field windings i2 and 18, respectively, which are energized through manually controllable potentiometer bridges I4 and 16, respectively, in accordance with the voltage drop across the commutating pole windings I8, I! and 28 of the main generator output circuit. The energizin circuit for the windings l2 and I8 may be traced from the positive terminal of the engine driven generator l2 through a resistor 16, a wire l1. and in parallel through a pair of variable resistors 18, 19 to the points and 8| on the potentiometer bridges I4 and 15, respectively, and hence through both sides or both potentiometers in parallel to a grounded wire 82. Thus the voltage appearing across each potentiometer H and 15 from the points 88 and BI to ground is proportional to the current in the generator output circuit as indicated by the voltage drop across the commutating pole windings i 8, l9 and 20. A suitable voltage for application to the motor exciter field windings l2 and I3 is selected on the potentiometers I4 and 15 by means of manually positionable steering handles 83 and 84, respectively.

It is desirable at this point to note certain mechanical features of the steering handles 88 and 84. Primarily these handles have for their purpose the determination of the standard of energization of the motor field windings I8 and II by selection from the potentiometers I4 and I5 of a suitable voltage proportional to line current for application to the motor exciter field windings I2 and I3. It will also be noted that the dead man interlock switches 53 and 54 previously mentioned are actuated by handle levers attached to the steering handles and that these switches are arranged to be closed as soon as the handles are grasped by an operator. Preferably, as indicated in the drawings, the steering handles 88 and 84 are biased to a normal position such that substantially full forward voltage is applied to the control field windings I2 and I3. The steering handles are also arranged to operate a number of auxiliary switches and rheostats. For example, the rheostats I8 and I8 are arranged to be fully in circuit when the steering handles 83 and 84 are in their normal positions and are connected respectively to the handles 83 and 84 for movement therewith to cut out resistance as the handles are drawn back toward the null points of the potentiometer bridges. The rheostats I8 and I8 are arranged to be fully cut out when the steering handles reach the null points. The purpose of these rheostats will be more fully described hereinafter in connection with automatic steering operation. In addition to the rheostats I8 and 18, the steering handles 88 and 84 are also arranged to actuate the .potentiometers 85 and 88 and the switches 81 and 88, respectively. The potentiometers 85, 88 and the switches 81, 88 are in the current limit circuit for steering control which will be described hereinafter, but for the present it will be suflicient to note that the sliders of the potentiometers 85 and 88 are normally at their maximum voltage positions and, since the forward halves of these ptentiometers are dead, the maximum voltage positions are maintained so long as the steering handles are forward of their null points. The potentiometers 85 and 88 become effective to reduce the tapped voltages as the steering handles are moved beyond their null points and into their reverse quadrants. The switches 81 and 88 are single-pole, double-throw switches which are normally closed on one pole so long as the steering handles are in their normal forward positions and are thrown to the other pole as soon as the connected steering handle is moved in the reverse direction a predetermined small distance fom the normal p ition.

It will be clear from the above description of the potentiometers I4 and 15 that with the steering handles 88 and 84 set in any predetermined position, the energization of the main control field windings I2 and I8 of the motor exciters I4 and I5 will vary in accordance with the magnitude of the main line current. Furthermore, since the exciters I4 and I5 serve as dynamoelectric amplifiers, it will be clear that the energization of the motor field windings I8 and 'II is proportional to the armature current of the motors, and that, except as modified under certain conditions explained hereinafter, the excitation characteristic of the traction motors I8 and I1 is similar to that of a series motor. It will be understood that the steering handles 83 and 84 are independently controllable so that the standard of excitation of each motor may be selected independently and set at any point between full forward field and full reverse field. The energization circuit for the motor exciter windings 12 and I3 is shown in simplified form at Fig. 4. While Fig. 4 shows only a single potentiometer bridge, it will be understood that the potentiometers I4 and I5 are connected in parallel circuit relation.

The series excitation characteristic of the traction motors l8 and I1 is desirable because it increases the speed up to which maximum utilization of available engine horsepower may be obtained. However, it has been found that even higher motor speeds than may be thus obtained are desirable at the point of maximum utilization of engine power. This demand arises in part from the fact that saturation of the motor fields at low vehicle speed in an effort to obtain maximum tractive effort for starting impairs to some extent the proportionality between line current and the motor fiux. Accordingly, in order further to increase the motor speed up to which maximum utilization of engine horsepower may be obtained, we provide additional means for exaggerating the series excitation characteristic of the motors by controlling the motor excitation in inverse proportion to vehicle speed as well as In direct proportion to the main line current. For this purpose we utilize the voltage of the tachometer generator 88 to energize, in proportion to vehicle speed, a pair of differential control field windings 88 and 8| on the motor exciters I4 and I5, respectively. Referring to Figs. 1 and 3, it will be observed that the control field windings 88 and 8| are connected in series circuit relation across intermediate points 82 and 88 of a bridge circuit, the line terminals 84 and 85 of which are connected to the terminals of the tachometer generator 88. Fig. 3 clearly shows the bridge circuit which comprises a pair of resistors 88 and 81 connected in series between the terminals 84 and 85 and a resistor 88 connected in series with a rectifier 88, also between the terminals 84 and The rectifier 88 is connected normally to 85. conduct, the terminal 84 being positive. The arms or the bridge are so proportioned that the point 82 is normally positive with respect to the point 88 and the motor e'xciter field windings 88, 8| are connected between these points. The energizing circuit for the windings 88 and 8| may be traced from the bridge terminal 82 through a rectifier I88 and the control field windings 88'. and 8| in series to the terminal 88. Thus, as the vehiclerspeed and tachometer voltage rise. the voltage across the bridge points 82, 88 Increases proportionately. As indicated at Fig. 1 the differential control field windings 88 and II are energized in opposition to the main field windings 12 and I3, respectively, so that. as the energization of the windings 88 and 8| increases with increasing vehicle speed, the net excitation of the traction motor exciters I4 and I5, and hence the excitation of the traction motors l8 and I1 themselves, is reduced in proportion to the speed of the vehicle. It will be borne in mind that this effect is superposed upon the reduction of tgaction motor excitation due to the decrease in main line current as the vehicle speed increases and produces an exaggerated series motor characteristic.

The reason for making use of the rectifier 88 as one arm of the bridge circuit of Fig. 3 will become evident from an examination of the curves of Figs. 6 and 7. The rectifier 88 serves as a nonlinear resistor which decreases its resistance as the voltage across the bridge increases. Referring first to Fig. 6, we have here shown the relation between vehicle speed and line current from zero speed up to maximum vehicle speed. The maximum vehicle speed is indicated at Fig. 6 by a line IOI. The manner in which the vehicle speed is limited to this maximum will be described hereinafter, but for the present it is sufficient to note that the broken line curve I02 of Fig. 6 indicates the manner in which the line current would diminish as the vehicle speed increases were it not for the additional field weakening due to the energization of the motor exciter field windings 90 and ill. Also at Fig. 6 the straight line I03 indicates the maximum current limit which, as has been heretofore explained, diminishes as the vehicle speed increases. As described in the preceding paragraph, the excitation of the control field windings 90 and 9| increases with vehicle speed and this increase is substantially linear over a portion I04 of the curve of Fig. 7, so that the line current of Fig. 6 follows a curve I rather than the curve I02. As indicated at Fig. 6, the departure of the curve I05' from the curve I02 becomes more pronounced as the energization of the field windings 00 and SI, as shown in Fig. 7, increases. Thus, due to the effect of the field windings 90 and iii, the line current begins to show an increase at a portion I05a of the curve I00, and begins to approach the current limit value. Since it is undesirable that the current limit be exceeded at this point, further increase in the,energization of the field windings 90 and II i prevented and, in fact, anet decrease in their energization may be produced by the action 01' the rectifier 99. The negative resistancevoltage characteristic of the rectifier 99 becomes pronounced at relatively high tachometer volttages, thereby to reduce the resistance of the bridge arm in which it lies and to preclude further increase 01' the voltage of bridge terminal 92 with respect to terminal 93 Indeed. the effect of the rectifier ll may be so pronounced as to actually reduce the voltage across the control field windings 00 and 0| despite further increases oi ta- .chometer voltage (see the portion I 04a of the,

curve I00 of Fig. 7) As indicated at Fig. 6, then, this has the eficctpi' again reducing theme current at a portion I0lb of the curve I05. By thus limiting, and in fact reducing, the energization thereof, exceeds the battery voltage. The spillover connection may be traced at Figs. land 3 from the positive terminal of the tachometer generator 60 through a wire I01, the rectifier I060, the wire 3+ and the battery 45 to ground, and from ground through the rectifier I06, a suitable visual or audible speed indicating device I08, the motor exciter control field windings 90 and 9 I, and the resistor 91 to the negative terminal of the tachomete generator I50. It will now be clear that the rectifier I00 is included in the normal or bridge excitation circuit of the field windings 90 and 9| for the purpose of preventing short circuitingof these field windings through the rectifier 99-up0n'splll-over in speed limit operation. From the above circuit it will be evident that the rectifiers I 06 and Mia are so arranged that current cannot fiow from the battery 45 through the tachometer generator 60. On the other hand, when the vehicle attains such a speed that the voltage at the output terminals or the tachometer generator attains a value greater than that of the battery voltage, these two blocking oi the windings 00 and BI in the high speed range,

an increase oi the line current to the limiting value is avoided.

Vehicle speed limit In addition to their function of exaggerating the series excitation characteristic of the traction motors, the motor exciter control field windings 00 and 9| are connected in circuit with the control battery 45 to provide a vehicle speed limit control and indication. For this purpose the voltage of the tachometer generator 00 is matched against the voltage of the battery 45 through a loop or spill-over circuit including the control field windings 90 and 0| oi' the motor exciters I4 and I5 and a pair 01' blocking rectifiers I06 and I060 (see Figs. 1 and 3), the rectifie I06a being in the rectifier bridge 50., The blocking rectifiers rectiilers will permit current to spill, over from the tachometer generator through the battery, the overspeed indicating device I08, and the motor exciter control field windings and SI. Furthermore, it is to be noted that the vehicle speed at which such spill-over will occuris substantially unaflected by changes in the battery voltage be- H cause of the fact that the tachometer generatorl is battery excited, so that as the battery voltage increases or decreases, the tachometer generator voltage increases or decreases proportionately.

The operation of the speed limit circuit may now be followed. It will be recalled that independently of any energization of the motor exciter control field windings 90 and SI through the speed limit spill-over circuit traced in the preceding paragraph, these field windings are continuously excited through the bridge circuit of Fig. 3 in the manner indicated at Fig. 7. Reierring now to Fig. 6, as the vehicle speed limit is approached the'llne current-is in the region I05b of the curve I05; It now the vehicle speed becomes such that the tachometer voltage causes current to spill over through the speed limit circuit thereby to energize the over-speed indicating device I00 and to superpose additional energization upon the normal energization of the motor exciter, field winding; 90 and 9i, the total energization of the windings 90 and 9| will increase sharply as indicated at the portion I04b of the curve of Fig. 7. Such increase in the energization of the difierential windings 00 and SI will cause a sharp decrease in the net motor exciter excitation, and hence in the net motor excitation, so that the line current will increase sharply as indicated at I050 of Fig. 6. This sharp increase in the line current will bring the line current to the limiting value at the speed of operation, as indicated by the curve I03 of Fig. 6, so that the current limit circuit previously described will be brought into operation. Thus with the motor current limited at the current limit value, the motor power is determined by the motor voltage. The motor voltage is proportional to the motor speed and to the excitation of the motor field windings, so that with the motor speed remaining at the maximum value and the motor excitation severely limited voltage is reduced below the spill-over value and supplementary excitation of the motor exciter fields 90 and Si ceases. It will thus be clear that the system will regulate on this point to maintain the vehicle speed at its maximum value.

The above operation of the system under speed limit conditions will be clarified by a consider ation of Fig. 5. This figure is a more or less conventional diagrammatic representation or the power characteristics of a Diesel-electric system. Thus the main generator i2, if driven at any selected speed, will exhibit an inherent full load volt-ampere characteristic of the general shape of the curve A, while the maximum power output of the engine III will be substantially constant as indicated by the curve B. Ordinarily the capacities of the various elements of the system are so selected that these curves intersect in the central operating range in order to attain maximum utilization of the available engine power over the widest possible range of vehicle speed. From the curves A and B it will be evident that within the central portion of the operating range the generator is demanding more power than the engine is capable of supplying and that the only way that the curves may be caused to coincide, as they must, is for the power output of the generatorto be slightly diminished by slight engine stalling within this range. Since such reduction in the engine speed also cuts down the available power output from the engine, it is desirabl if possible to cut down the generator.demand so that it just equal the engine output.

In the present system provision is made for reducing the generator excitation by means of the governor operated potentiometer 4G previously described. By way of example, the curves A and B of Fig. 5 have been drawn for the full open position of the engine throttle. With the throttle in this position the potentiometer 46 has efiected no change in the potential applied to the generator exciter field ll. However, as previously explained, the governor is provided with a certain degree of overtravel so that if, with the throttle in the full open position, the engine speed still cannot be maintained, the lever 29 attached to the governor moves the slide of the potentiometer 46 to effect a reduction in the energization of the generator exciter field winding 4| without further opening of the engine throttle, By thus reducing the excitation of the main generator l2, the power demand of the generator is reduced to a point where it just equals the available power output of the engine. This action is of a regulatory nature and results in bringing the generator voltampere characteristic into coincidence with the available power output of the engine over the constant power range C of the resultant generator volt-ampere characteristic shown in solid lines at Fig. 5. It will be understood that in the region D the voltage of the generator is limited to a predetermined maximum value by saturation of the generator field poles. The portion E on the curve of Fig. 5 is determined by operation of the current limit circuit at zero or low vehicle speed. It will be understood that as the vehicle speed increases, the current limit is reduced as indicated by the constant current lines E1 and E2 of Fig. 5. The portion F of the generator voltampere characteristic represents the pure resistance drop through the armatures of the traction motors along which the line current builds up in proportion to generator voltage when the vehicle starts from standstill.

Referring again to the operation of the vehicle speed limit with the curves of Fig. 5 in mind, it will be evident that, if the vehicle is operating at a point I H) on the generator volt-ampere characteristic when the vehicle speed reaches its maximum value, the sharp increase in motor exciter difierential field excitation produced by the speed limit circuit and indicated at Fig. '7, and the consequent sharp increase in line current indicated at Fig. 6, will cause the operating point H0 of Fig. 5 to move down the curve C to the maximum speed current limit curve E2, and hence down the current limit curve E2 to a point such as Ill of Fig. 5. It will be clear that the system power represented at the point I I I is less than that represented by the constant power curve C so that the power output of the motors will be reduced and the vehicle will slow down as previously described.

Stabilization of motor excitation It will be understood from the above that when the vehicle is moving forward under power, the counter-electromotiveforce of the motors is almost equal and opposite to the output voltage of the engine driven generator l2. Since the motor voltage is proportional to the product of the motor flux and motor speed, it is apparent that when the vehicle is traveling at a relatively high speed the motor flux is quite weak while the generator I2 is operating under partial saturation in its upper range of voltage. Under such conditions it will be evident that the motor exciters H and I5 are capable of forcing an extremely rapid change or motor field flux with onl a relativel small change in motor field energization, while because of the generator field saturation, a relatively large voltage change at the generator exciter I3 is necessary to produce a proportional change in the generator field flux. Accordingly, if the excitation of the traction motors is suddenly reduced or reversed to initiate dynamic braking operation, it will be clear that a transient current of considerable magnitude might flow in the generator output circuit because of the fact that the motor field flux will reverse much more quickly than the generator flux can be reduced by operation of the current limit circuit. For the purpose of bringing this undesirable condition within permissible limits, the motor exciters l4 and I5 are provided with stabilizing control field windings H5 and H6, respectively. Referring to the drawings, it will be observed that the positive terminal of the motor exciter l5 and the negative terminal of the motor exciter M are connected together and to ground through a wire I ll, while the positive terminal of the motor exciter i4 is connected through the stabilizing control field winding H5, a condenser H8, and the stabilizing control field winding H6 to the negative terminal of the motor exciter l5. Thus when the system is operating under steady-state conditions, no current flows through the stabilizing windings H5 and H6, the condenser H8 merely being charged to a potential equal to the sum of the potentials across the output terminals of the motor exciters I4 and 15. The stabilizing circuit is a loop circuit comprising the armatures of the motor exciters l4 and IS, the control field windings H5 and H6, and the condenser ll8, all in series circuit relation, the circuit being tied to ground between the armatures of the motor exciters by the wire lll.

It may now be observed that if the voltages across the output terminals of both motor exciters are changing in the same sense, that is, if increased or decreased motoring or braking torque is simultaneously being called for on both exciters, or if the excitation of both exciters is suddenly and simultaneously changed from forward excitation to reverse excitation to shift from motoring to braking operation, the condenser III is charged or discharged (depending upon the direction of the change) through the stabilizing control field windings Ill and HE. These stabilizing windings are so arranged that the components of field excitation which they produce under such conditions of change in the motor exciters l4 and I! tend to oppose the change of voltage in each machine. In this manner the rate or change of output voltage of the motor exciters is sufllcientl decreased so that the generator exciter difi'erential field III is able to make the fiux of the generator field 3| follow the motor fiux sufiiciently closely to pre-' vent an exceedingl high transient current during a rapid change of motor excitation. It, on the other hand, the voltage of only one of the motor exciters is changed, either by increasing or decreasing the field excitation of that motor while the voltage of the other motor exciter is maintained constant, the condenser H8 will also undergo a change in charge. In this case however the current through the stabilizing field windings ill and III will be in the same direction in both windings, so that the current through the stabilizing winding of that motor exciter whose voltage is undergoing change will tend to oppose the change, while the current through the stabilizing winding of that motor exciter whose field excitation is not otherwise undergoing change will tend to produce a change in voltage of that exciter in a direction opposite to the voltage change in the first exciter. This lattereifect is desirable in connection with steering operation, as will appear hereinafter. Steering is eflected by moving one steering handie to change the, torque of the associated motor. In order that maximum steering efiiciency may be eflected, means are preferably provided for automaticall changing the torque of the other motor in the opposite direction without necessitating movement of the other steering handle. It will be clear that the above transient efl'ect aids such a change.

Steering As briefly mentioned in the foregoing paragraph, steering of the vehicle is effected by actuating one or the other of the steering handles to reduce or reverse the torque of the associated motor without similarly reducing the torque of the other motor, thereby causing the vehicle to turn toward the side upon which the torque is reduced or reversed. Referring to Fig. 1, if the steering handles are in the full forward motoring position shown, steering may be efiected by pulling one handle, for example the right-hand steering handle 84, backward toward or to the null position. If very abrupt steering is required, the handle may be pulled back into the reverse quadrant to effect the reversal of torque and braking operation of the right-hand motor. During such operation the left-hand motor continues to exert a forward motoring torque. It will of course be understood that if the vehicle is traveling in the reverse direction, steering is effected by moving one handle toward or into the forward quadrant.

' quadrant while the In order to efiect steering with a minimum of disturbance or the main line current, it is desirable to maintain constant the net motor voltage during steering operations. Means are therefore provided for automatically increasing the excitation and hence the torque of the motor whose steering handle is not disturbed when the voltage and torque of the other motor is decreased. For example, if the steering handle I is drawn toward the null point of the potentiometer 15 while the steering handle 83 is left in the position shown, the resistance of the rheostat 19 will be gradually diminished and will be completely out out when the handle ll reaches the null point. Referring now to Fig. 4, it will be evident that as the rheostat 19 is cut out, the voltage applied to both double potentiometers I4 and 15 is gradually increased. Thus, since the steering handle I! is not moved, it will be clear that the field excitation of the motor exciter II is increased, thereby to increase the voltage and torque of the motor 16. Of course, the voltage applied to the potentiometer I4 is also increased, but the reverse movement of the handle ll takes precedence so that'the resultant excitation of the motor exciter i5 is decreased, thereby decreasing the voltage and torque of the motor II. By suitable proportioning of the various elements of the system, the rheostat 19 may be caused to increase the voltage of the motor It by the same amount that the voltage of the motor I! is decreased by the steering handle ll until the null point is reached and the rheostat I! is completely out out. This condition is illustrated at Fig. 8 which shows the relation between steering handle travel and motor voltage for the condition assumed. It will be clear that if the handle 03 is moved and the handle 84 left stationary. the rheostat II will have the effect of increasing the voltage of the motor l1.

Control of current limit by steering To efi'ect severe steering operation, one of the steering handles may be pulled into the reverse other handle is left in the forward quadrant of the associated potentiometers 14 and II. For added maneuverability of the-vehicle, the response to such a demand for abrupt steering is enhanced by providing a temporary increase in the forward torque upon the forward running track and an increase in the braking torque temporary increase in the torque is effected by boosting the current limit above the normal current limit value for the existing speed of operation in response to positioning of the steering handles in opposite quadrants.

Continuing with the example assumed, let it be supposed that the steering handle 84 is moved into the reverse quadrant. Fig. 2, it will be evident from the foregoing description of the apparatus connected to the steering handle 84 that the selector switch 88 will have been thrown into engagement with the contact I 2| as soon as the steering handle 84 was moved backward a predetermined small distance from its normal position, thereby to connect the terminal 61 of the rectifier bridge 58 to the tap 65 of the tachometer potentiometer 64 through the slider of the steering handle potentiometer 88, rather than .through the slider of the steerupon the reversed track. Such difl'erential or steering Referring now to.

eter 88 is effective only in reverse quadrant. However, as soon as the steering handle is moved into the reverse quadrant, the slider of the potentiometer 86 moves over the live section of the potentiometer, thereby gradually to reduce the tachometer voltage applied between the terzminals 58 and 81 of the rectifier bridge 58. This reduction of voltage applied to the terminals. and 81 of the rectifier bridge 58 is effected by reason of the fact that the steering handle potentiometer 88 is connected in parallel circuit relation with that portion of the tachometer potentiometer 68 between the taps 85 and 85a. As the slider of the potentiometer 88 is moved in a clockwise direction as viewed at Fig. 2 toward its extreme reverse quadrant position, the voltage applied to the terminal 81 of the rectifier bridge 58 changed from that of the tap 85 on the potentiometer 84 to that of the tap 85a, thereby to reduce the differential voltage between the terminals 86 and 81. As the voltage between the terminals 86 and 81 is reduced, the unidirectional voltage between the terminals 82 and 88 of the rectifier bridge 58 is correspondingly reduced. It will be recalled from the foregoing description of the current limit circuit that the rectified tachometer voltage between the terminals 82 and 83 of the rectifier bridge 58 is connected in aiding relation to the voltage drop across the resistor 81 to oppose the voltage of the battery 45. It will be clear that as the rectified tachometer voltage is reduced by the action of the steering handle potentiometer 88, a progressively higher voltage drop across the resistor 51 is required to break down the rectiflers in the bridge 58 and produce current limit spill over through the battery 45.

It will be understood that, if the power demand is such that the vehicle is operating at the current limit, the temporary increase of current limit value resulting from movement of only the steering handle 84 into the reverse quadrant will increase both the forward torque on the motor l8 and the reversed or braking torque upon the motor 11. This increase in torque is occasioned primarily by the increased current, and to a lesser extent by increasing motor voltages.

Referring now to Fig. 8, it will be observed that the temporary boosting of the current limit beyond the null point permits a slight increase in the voltage of the motor l6 due to increased excitation of the current responsive exciter field winding 12. Increased excitation of the winding 12 is very slight, however, due to the limiting effect of a shunt circuit for the potentiometers 14 and 15 shown at Fig. 4 and to be described hereinafter. The limiting shunt circuit comes into operation soon after the actuated steering handle passes the null point and thereafter maintains substantially constant the excitation of the motor 16. Beyond the null point of the steering handle 84, however, the voltage of the motor [1 increases in a negative sense at a more rapid rate than that at which it decreased from its initial positive value to zero. This increased rate of change of voltage of the motor 11 is due to the fact that when the steering handle 84 passes the null point the rate of change of voltage of the motor I1 is due solely to movement of the steering handle 88, the potentiometer 18 now being ineffective to increase the voltage applied to the steering handle potentiometer 15. When the voltage of the motor 11 was positive such increasing applied voltage opposed the effect of steering handle movement.

asoapao It will be understood that if, instead of moving the steering handle 88, the operator had moved the steering handle 88 into the reverse quadrant and left the steering handle 84 stationary, the potentiometer 85 would have produced the same effects as just described in connection with the potentiometer 88. In this connection, it will be observed from Fig. 2 that by such movement the switch 88 remains in the positon shown, while the switch 81 is thrown into engagement with its fixed contact I28. Movement of the switch 81 has no eifect, so that the terminal 81 of the rectifier bridge 58 remains connected to the tap 85 of the potentiometer 88 through the slider of the steering handle potentiometer 85. Thus, as the steering handle potentiometer 85 moves past the null point into the reverse quadrant, the tachometer voltage applied to the rectifier bridge 58 is reduced in the manner heretofore described.

Furthermore, if both steering handles are simultaneously moved into their reverse quadrants to effect braking operation without steering, both selector switches 81 and 88 are actuated; that is, the switch 88 engages the contact HI and the switch 81 engages the contact I28, thereby completely to disconnect the steering handle potentiometers 85 and 88 and to connect the terminal 81 of the rectifier bridge directly to the 0 tap 65 of the tachometer potentiometer 88.

Thus, when the steering handles 83 and 84 are moved in unison, the steering handle potentiometers 85 and 88 are rendered ineffective to control the current limit value.

Dynamic braking operation Dynamic braking op ration without steering is effected by drawing both steering handles 88 and 84 simultaneously from their forward quadrants into their reverse quadrants. During this operation the potentiometers 85 and 88 are ineffective to raise the current limit because they are by-passed by the switches 81 and 88 as expiained in the preceding paragraph.

However, the rheostats 18 and 18 are effective, so long as the steering handles are still in their forward quadrants, to increase the volta e applied to the motor field windings 18 and 11, as explained heretofore in connection with the steering operation. By thus increasing the standard of excitation of the motor fields, a high braking torque is maintained at relatively low line currents.

While the braking torque at low line currents is maintained in the manner described above, means are also provided for limiting the braking torque at high line currents. The need for such a limitation will be appreciated when it is realized that under braking conditions the field excitation of the traction motors l8 and I1 is reversed so that these motors act as generators, their voltages adding to the voltage of the main engine driven generator 12. Under these conditions the series excitation characteristic of the traction motors becomes a series generator characteristic having a marked cumulative effect; that is, as the excitation of the traction motors is increased in a braking sense, the line current is increased and as the line current increases, the excitation of the traction motors is further increased. This effect is aggravated by the transient current discussed above in connection with the motor exciter stabilizing windings. It will be recalled that transients due to the inability of the generator to reduce its voltage as quickly as the motor voltage is reduced are apt to cause temporary overshooting of the current limit. While the motor exciter stabilizing windings H and IIS limit this overshooting to permissible values, it is also desirable to limit the braking torque when such high line currents exist. Indeed, it has been found desirable to limit the braking torque at high line current even though the line current does not exceed the current limit value. For this purpose we provide means for limiting the excitation of the traction motors I6 and I! to a predetermined maximum value so that the series excitation characteristic of the traction motors is transferred to a constant or shunt excitation characteristic upon the occurrence of a predetermined high line current. Fig. 4 shows the manner in which the energization of the main motor exciter control field windings I2 and I3 is limited to a definite maximum value. As has been previously pointed out, Fig. 4 shows the potentiometer bridge I4 (the potentiometer bridge I! being connected in parallel circuit relation) connected across the interpole windings II, I9 and 20 in the generator output circuit in series with the resistor 16 and the rheostats I! and 19. Furthermore, it will be now understood that normally the basic series characteristic of .the motor exciter winding 12 arises from the fact that the potential at the point 80 of the potentiometer I4 is proportional to the voltage drop across the commutating pole windings ll, I9 and 20 as derived through the resistor I8, 18 and 19. As explained heretofore, this proportionality is modified to some extent in braking operation by reduction of the resistance of the rheostats II and 19. However, in order to limit the potential at the point OI to a predetermined maximum value, this point is tied through a blocking rectifier I28 to a point of intermediate voltage I21 on the battery 45. From Fig. 1 it will be evident that the point I21 has a fixed potential slightly above ground potential as determined by the battery 45. So long as the potential of the point 80 remains below that of the point I21 on the battery 45, current cannot fiow from the battery through either the potentiometer or the commutating pole windings I8, I9 and 20 because of the presence of the blocking rectifier I 26. However, should the line current through the commutating pole windings I8, I9 and 20 attain such a value that the potential at the point 80 tends to attain a value in excess of the potential at the point I21, current will spill .over from the point 80 through the rectifier I28 and the connected portion of the battery 45 to ground, thereby limiting the potential of the point 00 to a predetermined maximum value. After this operation the potential of the point 80 remains substantially fixed regardless of the value to which the line current, and hence the potential drop across the commutating pole windings I8, I9 and 20, may go. Therefore, when the line current attains a value sufiicient to cause such spill-over, the energization of the motor exciter control field windings I2 and I3 and accordingly the excitation of the traction motors I8 and I1, attains a fixed maximum value and is possessed of a shunt characteristic. In this manner the undesirable cumulative effects of motor field strength are minimized when going into dynamic braking operation. It will be understood that the excessive potential at the point 80 may not be due alone to a simple resistance drop across the commutating pole windings I8, is and 20, but that upon a very rapid change in line current, as due to a quick reversal of the traction motor field windings, such an excessive potential may arise in part from the inductive drop across the commutating pole windings. The spill-over circuit through the rectifier I20 also guards against such excessive inductive transient potentials.

Under conditions of rapidly changing line current in suddenly going into braking operation as described above, the inductive potential appearing across the interpole field windings I8, It and 20 is utilized to hasten the decrease in excitation of the generator I2 to its steady-state value. For this purpose the energization of the differential field winding III of the generator exciter I 3 is arranged to go to exceedingly high values. It will be recalled that the diflerential field winding ll of the exciter I3 is energized across the commutating pole windings I8, I9 and 20 so that any inductive voltage drop appearing at the positive terminal of the generator I2 will provide a transient increase in energization of the winding 40 and hence a rapid decrease in the net excitation of the generator exciter II and the generator I2. By forcing an increased current through the diflerential field winding 40, the inductive voltage at the positive terminal of the generator I2 also appears across the resistor 51 in the generator exciter difl'erential excitation circuit and tends to bring into operation the line current limit circuit previously described, so that the energization of the generator exciter diiferential control field I may be further increased by spill-over of the current limit circuit through the rectifier bridge 58 and the battery 48, as previously described.

Operation In view of the foregoing detailed description of the various parts of the system and of their manner of connection and mode of operation, the operation of the system as a whole will be readily understood from the following brief description from the viewpoint of the operator.

Three simple and convenient controls are provided at the driver's position, namely, the accelerator pedal 26 for controlling the speed and power output of the internal combustion engine III, and the left and right steering handles 83 and 84, respectively, for independently controlling the speed, torque, and direction of rotation of the left and right-hand tracks. As previously described, the steering handles 83 and 84 are providedwith dead man interlocks 53 and 64 normally biased to an open position.

Assuming first that the vehicle is at standstill and that the engine I0 is running at idling speed, motoring operation is initiated simply by grasping one or both of the steering handles to close one or both of the dead man interlocks 53, 54. The interlocks 53, 54 are connected in parallel circuit relation, and closure of either interlock completes an energizing circuit for the main control field winding ll of the generator exciter II through a circuit which has been previously traced. Simultaneously an energizing circuit is completed from the positive terminal of the battery 45 through an actuating coil I30 oi the throttle solenoid 31 to the wire 52 and through the dead man interlocks 53 and B4 to ground. This circuit also includes a normally closed interlock contact III on an overspeed device I32. The overspeed device I32 is arranged to be actuated upon overspeeding oi the engine I0 to deenergize the throttle solenoid 31. As has been explained hereinbefore, actuation of the throttle solenoid 31 adjusts the setting of the governor 25 to maintain a predetermined minimum running speed of the engine ill, thereby to assure adequate power for steering or braking even though the accelerating pedal 26 should be accidentally r :leased.

Closure of one of the dead man interlocks 53, 54 to initiate motoring operation also completes an energizing circuit through a reverse power relay RPR for an actuating coil i3! the braking contactor B. The braking contactor B is normally open, but is closed during motoring operation to shunt the braking resistor II in the main generator output circuit. The reverse power relay RPR is provided with a polarizing winding I36 and a directional winding I31. Upon closure 0! one of the dead man interlocks II or 54, an energizing circuit is completed for the polarizing winding I38 from the wire 13-!- through a normally closed interlock contact [3. on the braking contactor B and the winding I36 to the wire 52 and hence to ground. When so energized, the reverse power relay RPR picks up and is retained in its actuated position by a holding current of limited value through. a resistor III which shunts the normally closed interlock contact I". Upon actuation of the relay RPR an energizing circuit is completed for the actuating winding II! of the braking contactor B through a circuit which may be traced from the wire B+ through a normally open interlock contact I" on the reverse power relay RPR, the actuating winding ll! of the braking contactor B, a normally closed interlock contact Hi on the braking contactor B, and a normally closed interlock contact I42 on a time delay relay TDR to ground. Thus energized the braking contactor B picks up to close the shunting contact 22 around the braking resistor II and to complete for its actuating coil I35 a holding circuit through a current limiting resistor Ill and a self-actuated normally open interlock contact I. After the contactor B picks up it closes a circuit for an actuating coil I55 of the time delay relay TDR through the interlock contact 49 on the contactor B. Opening of the time delay relay contact 2 has no effect for the present because it is now shunted by the lock-in contact i on the contactor B.

By way of summary then, simple closure of one or the other of the dead man interlock contacts 53 or 54 immediately increases the engine speed to a predetermined minimum, picks up the braking contactor B to shunt the braking resistor 2i, and completes a field excitation circuit for the generator exciter l3. The consequent current flowing in the output circuit of the generator I! produces a voltage drop across the commutating field windings i8, i9 and 20 and thus energizes the main control field windings 12 and 13 of the traction motor exciters ll and i through the potentiometer bridges 1| and 15, respectively, so that a torque is supplied to the driving wheels. Since the steering handles 83 and 84 are in their position of maximum forward torque, the vehicle will begin to accelerate from standstill in the forward direction. As the vehicle comes up to speed, the motor current falls of! and the voltage rises from a point I30 on the IR line F of Fig. 5 and along a low engine power curve C to some point such as the point Hi. It will be understood that as the line current diminishes, the motor torque also diminishes so that the point i3l is determined as that point at which the total motor torque just overcomes the resistance to motion of the vehicle.

With the establishment of a current in the generator output circuit, the directional windaaeaeao ing ll! of the reverse power relay RPR is energized in proportion to the motor voltage and cumulatively with respect to the polarizing wlnd ing I" of the reverse power relay. The winding I31 thus tends to hold the reverse power relay in its actuated position.

To increase the speed of the vehicle the accelerator pedal 26 must be depressed. The englue in is thereby speeded up and its power increased, thus increasing the generator output current and the motor torque. The vehicle will therefore speed up until the increased motor torque just balances the resistance of the terrain over which the vehicle is operating. It should be noted that the speed of the vehicle cannot be increased simply by moving the steering handles forward (assuming that they had previously been removed from the maximum forward position) to increase the motor excitation. This will be evident from a consideration of the fact that no greater power is thereby supplied to the motor since the generator power output is not changed by moving the steering handles. The only effect of moving the steering handles forward simultaneously is to increase the motor voltage and thus decrease the motor current so that the operating point moves along one of the constant horsepower curves such as that shown in Fig. 5 to a higher voltage point.

If during forward motoring operation the sccelerator pedal is depressed sufllciently far to increase the vehicle speed to its predetermined maximum value, the speed limit circuit functions in the manner heretofore described to diminish the excitation of the traction motors and to give a visual or audible indication at the overspeed indicating device II. The operation of the speed limit circuit and its relation to the current limit feature has been explained hereinbefore. So far as the operator is concerned the functioning of the speed limit circuit and an indication on the overspeed device I08 should be taken as a signal for releasing the accelerator pedal to some extent, thereby to reduce the power output of the engine and to permit the system operating point to move off of the current limit. This is desirable, as obviously it is more efficient to operate the system at low current and high voltage than to operate at low voltage at high current.

An outstanding feature of the described arrangement is that, in the event that the vehicle while motoring forward encounters a downhill grade and the operator makes no attempt to check the vehicle speed, the speed limit protection will operate not only to reduce the engine horsepower output in the manner previously explained but will actually reverse the net excitation of the traction motor field windings thereby automatically to initiate dynamic braking operation and to maintain the vehicle at the maximum permissible speed. To illustrate, let it be assumed that the vehicle is brought to a steep downhill grade with the engine in running at its maximum speed and the steering levers 83 and 84 pushed forward to their maximum torque positions as shown at Fig. i. As the vehicle speed increases the voltage of the tachometer generator 6|) will become so great that the current spilled over from the tachometer generator through the battery 45 and e motor exciter differential windings and 9 will become sufficient completely to overpower the main control field windings l2 and I3 and to reverse the aaoaoao net excitation of the motor exciters I4 and I and the motors I6 and I1 thereby to initiate dynamic braking operation.

The manner in which the vehicle may be steered by reducing or reversi g the torque on one or the other of the mole; has been previously described. Also, dyna c braking without steering by simultaneous transfer of the steering handles 83 and 84 to their reverse quadrants has been explained with reference to the motor field excitation circuit. It may now be noted that when such dynamic braking operation is initiated the back electromotive force of the traction motors I6 and I1 is reduced to zero and then reversed. As the back electromotive force of the traction motors approaches zero or begins to reverse in going into braking the directional coil I31 of the reverse power relay RPR. functions to drop out the reverse power relay thereby to deenergize the actuating coil I35 of the braking contactor B and to cause the contactor B to drop out and unshunt the braking resistor 2|. During dynamic braking operation the power generated by the generator I2 and the traction motors I6 and I1 is absorbed in the braking resistor. Proper operation of the reverse power relay RPR is ensured by a pair of capacitors I50 and I 5| connected across a resistor I52 in the circuit of the directional coil I3I. These capacitors function to cause the reverse power relay to anticipate its normal setting sufllciently so that even when the rate of change of motor voltage is greatest the braking contactor B is opened at the desired time. As braking proeeeds with the motor voltage reversed the reverse power relay RPR remains dropped out and the braking contactor B likewise remains deenergized. As the motor voltage decreases with decreasing vehicle speed the voltage of the main generator I2 increases to maintain the continuing unidirectional current in the main line circuit. Since the current in both the motor armatures and the motor field windings is maintained, dynamic braking may be made efiective until the vehicle reaches standstill.

Opening of the braking contactor B indicating that dynamic braking is in progress also effects a number of protective functions necessitated by braking operation. In the first place the interlock contact 59 01' the contactor B disables the current limit circuit thereby to remove the current limit effect during dynamic braking. This will be evident from Fig. 2, and is necessitated by the fact that the voltage of the engine driven generator I2 cannot be permitted to reverse under the influence of the differential exciter field winding 40 in an attempt to maintain the line current at the current limit value. If the generator voltage were permitted to reverse it would mean that the generator was acting as a motor to drive the internal combustion engine I0. While slight overspeeding of the engine I0 is permissible, any substantial overspeeding must be avoided in order to prevent damage to the engine. A second normally closed interlock I53 is also included on the braking contactor B for the same reason. When the braking contactor drops out the interlock I53 shunts the resistor 55 to insure that maximum forward field is maintained on the generator exciter I3. Unless the generator exciter forward field H is kept at its maximum value during braking it is possible that even the minimum strength of the enerator differential field 4| would be sufllcient to cause the reversal of the main generator voltage and overspeeding of the engine II).

It is also to be noted that during braking the interlock contact I38 on the contactor B is closed to shunt the holding resistor I39 in circuit with the reverse power relay polarizing coil I38 thereby to change the calibration of the reverse power relay and to restore its normal pickup circuit. The interlock contact I H on the braking contactor B also closes during dynamic braking to short circuit the holding resistor I43 in series with the actuating coil I35 01' the braking contactor and to restore the normal pickup calibration of the braking contactor.

During dynamic braking operation while the reverse power relay RPR remains dropped out energization of the braking contactor coil I35 is precluded by the time delay relay 'IDR, the actuating winding I55 of which is maintained energized through a normally closed interlock contact I55 on the reverse power relay RPR which shunts the now open interlock contact 49 on the contactor B. When dynamic braking is completed and the vehicle speed is reduced to zero, the RPR relay recognizes this fact by the fact that the motor voltage Is substantially zero. The reverse power relay then picks up and deenergizes the forward field winding 4I of the generator exciter I3 at the RPR interlock contact I56. Opening of the contact I56 also deenergizes the time delay relay 'I'DR. The time delay relay, however, does not immediately drop out. Thus for a short time th! generator exciter I3 is energized only by its differential field winding 40 and its voltage is rapidly brought to zero. After a suitable time which is just sufllcient to allow the line current to come approximately to zero, the time delay relay TDR drops out and closes its interlock contact I42. Closure of the contact I42 completes an energizing circuit for the braking contactor B and contactor B picks up and locks itself in through its interlock contact I44. It will be understood, of course, that at this time the operator will release the dead man handle interlocks 53 and 54 and return the steering handles 83 and 34 to their biased normal position. If the operator retains the handles in the braking position, the reenergization of the braking contactor B will complete an energizing circuit for the generator forward field windin through its interlock contact 43 and initiate reverse motoring operation.

Referring now to Fig. 4 the purpose of a blocking rectifier I60, not previously mentioned, will become evident. The rectifier I60 is located between the points and 8| of the potentiometer bridges I4 and I5 and ground,and has for its purpose the prevention of a reverse current in the main line circuit. Let us assume, for example, that the steering handles 83 and 84 are moved to braking position with the dead man handle interlocks 53 and 54 open. Under these conditions the forward field H of the generator exciter I3 would be disabled so that there would be nothing to determine the direction of current flow in the generator output circuit. A reverse line current can start in the system, and under such conditions the reverse power relay RPR. would pick up under the influence of the winding I31 alone and cause closure of the braking contactor B. Under such conditions the traction motors I6 and I1 would build up as series generators as desired but without the braking resistor 2| in the main line circuit, so that all the power would be absorbed in the generator I2 thereby causing the generator to overspeed the engine Hi. The blocking rectifier (60 serves to prevent the motors from receiving any excitation due to a reversed line current, so that braking under such conditions is not possible. Thus at Fig. 4 it will be evident that if the potential of the grounded wire exceeds the potential of the point 8i, as due to a reversal of line current, the potentiometers i4 and will be short circuited through the blocking rectifier Hill.

From the foregoing it will be clear that we have provided an extremely fiexible electric power system and motor control system of general application which, when used in its entirety, possesses particularly attractive features for application to a track laying vehicle. The system is remarkable for its speed of response and for its absolute protection against overload without regard to the severity of requirements imposed upon it by the operator. These virtues, and particularly the speed of response, are due primarily to the refined limiting and control circuits applied in conjunction with an electric power system in which a continuing controlled unidirectional current of limited magnitude is maintained at all times in a motor armature circuit during both motoring and dynamic braking operations while the speed, torque, and direction of rotation of each motor are independently controlled through its low power field excitation circuit.

While we have described only one preferred embodiment of our invention by way of illustration it will be evident that many modifications will occur to those skilled in the art and that we, therefore, intend by the appended claims to cover all such modifications as fall within the true spirit and scope of our invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

i. A motor control system comprising a direct current generator arranged to supply current to an output circuit, a pair of direct current motors having armatures and field exciting windings, said armatures being connected in series circuit relation in said output circuit, excitation means for said generator, means responsive to the current in said output circuit for controlling the excitation of said generator to limit said current to a selectable maximum value, separate manually controllable means for supplying independently variable excitation currents to said motor field exciting windings thereby separately to control the magnitude and direction of motor torques, and control means automatically operable upon movement of one of said manually controllable means without like movement of the other said means to control said maximum current limit value in accordance with such movement.

2, A motor control system for steering an electrically driven vehicle having laterally spaced driving elements comprising an electric traction motor connected to drive each of said driving elements, each of said motors including an armature and a field exciting winding, a direct current generator connected to supply current to said motor armatures in series circuit relation, excitation means for said generator, means responsive to the current in said motor armatures for controlling the excitation of said generator to limit said current to a selectable maximum value means for energizing said motor field windings to excite said motors, separate manually operable means movable to vary and reasoaeao verse independently the energization of said motor field exciting windings thereby separately to control the magnitude and direction of motor torques, and control means automatically operable upon response to demand for a predetermined minimum diilerential torque between said motors to increase said differential torque thereby to enhance the steering response of said vehicle.

3. A motor control system comprising a direct current generator arranged to supply current to an output circuit, a pair of direct currentmotors having armatures and field exciting windings, said armatures being connected in series circuit relation in said output circuit, excitation means for said generator, means responsive to the current in said output circuit for controlling the excitation of said generator to limit said current to a selectable maximum value, means for energizing said motor field exciting windings to excite said motors, separate manually operable means movable to vary independently the energization of said field exciting windings thereby separately to control the magnitude and direction of motor torques, control means automatically operable upon movement of only one of said manually operable means beyond a predetermined point in its travel to control said maximum current limit value, and means actuable in response to movement of both said manually operable means beyond said predetermined points for rendering said control means ineffective to control said current limit value.

4. .A motor control system comprising a direct current generator arranged to supply current to an output circuit, a pair or direct current motors having their armatures connected in said output circuit, excitation means for said generator, separate manually controllable,means for supplying independently variable excitation currents to said motors, means responsive to the current in said output circuit for reducing the excitation of said generator to limit the current in said circuit to a selectable maximum value, and control mean operable upon movement of only one of said manually controllable means beyond a predetermined point in its travel automatically to increase said maximum current value.

5. A motor control system comprising a direct current generator arranged to supply current to an output circuit, a pair of direct current motors having their armatures connected in series circuit relation in said output circuit, excitation means for said generator, separate manually controllable means for supplying independently variable excitation currents to said motors thereby separately to control the motor torques, and means automatically responsive to a demand for a predetermined diflerential between said motor excitation currents to increase the torque dinerential between said motors.

6. A motor control system comprising a direct current generator arranged to supply current to an output circuit, a pair of direct current motors having their armatures connected in series circuit relation in said output circuit, excitation means i or said generator, separate manually controllable means for supplying independently variable excitation currents to said motors thereby separately to control the motor torques, means responsive to the current in said output circuit for reducing the excitation 01 said generator to limit the current in said circuit to a selectable maximum value, and control means rendered operable in response to a demand for a predetermined differential torque between said motors for increasing said current limit value thereby to increase said differential torque.

7. A motor control system comprising a direct current generator arranged to supply current to an output circuit, a pair of direct current motors having their armatures connected in said output circuit, excitation means for said generator, separate manually controllable means for supplying independently variable excitation currents to said motors, means responsive to the current in said output circuit for reducing the excitation of said generator to limit the current in said circuit to a selectable maximum value, control means operable upon movement of only one of said manually controllable means to reverse the excitation of the associated motor for increasing said maximum current limit value, and means responsive to similar movement of both said manually controllable means for rendering said control means inei'i'ective to control said maximum current value.

8. A motor control system comprising a, direct current generator arranged to supply current to an output circuit, a pair of direct current motors having their armatures connected in series circuit relation in said output circuit, excitation means for said generator, separate manually controllable means for supplying independently variable and reversible excitation currents to said motors thereby separately to control the magnitude and direction of the motor torques, and

means automatically operable upon reversal of,

the torque of only one of said motors to increase the diiierential torque between said motors.

9. A motor control system for an electrically driven vehicle provided with laterally spaced driving elements comprising a direct current generator, arranged to supply cuit, an electric traction motor drivingly connected to each of said driving elements, each of said motors including an armature and a field winding, means for connecting said armatures in series circuit relation in said output circuit, excitation means for said generator, separate manually controllable means for supplying independently variable excitation currents to said motors thereby separately to control the magnitude and direction or motor torques, and means responsive to reversal of the torque of only one of said motors for increasing the differential torque between said motors thereby to enhance the steering response of said vehicle.

10. A motor control system comprising a direct current generator connected to suppl current to an output circuit, a pair of direct current motors having their armatures connected in series circuit relation in said output circuit, differential excitation means for controlling the excitation of said generator in accordance with the current in said output circuit, said differential excitation means comprising a current responsive element and a resistor connected in series circuit relation across a portion of said output circuit. means for limiting the current in said output circuit toa selectable maximum value comprising a unidirectionally conducting shunt circuit for said resistor including a source of bias potential disposed in said shunt circuit to oppose the voltage drop across said resistor, separate manually controllable means for supplying independently variable and reversible excitation currents to said motors, and control means operable upon movement of either of said manually movable means beyond a predetermined point in its travel for controlling said current to an output cirsource of bias potential to increase said maximum current limit value.

11. A motor control system comprising a direct current generator arranged to supply current to an output circuit, a pair of direct current motors having their armatures connected in series circuit relation in said output circuit, a field exciting winding for said generator, an auxiliary dynamoelectric machine connected to supply excitation current to said field exciting winding, means for reducing the excitation of said generator in accordance with the current in said output circuit comprising a differential field exciting winding for said dynamoelectric machine connected in series circuit relation with a current limiting resistor across a portion of said output circuit, means for limiting the current in said output circuit to a selected maximum value comprising a source of substantially constant bias potential connected in a unidirectionally conducting shunt circuit across said resistor to oppose the voltage drop across said resistor, separate manually controllable means for supplying independently variable excitation currents to said motors, means for selecting the current limit in said output circuit comprising a second source of bias potential connected in said unidirectionally conducting circuit in aiding relation to the voltage drop across said resistor, and control means operable upon movement of only one of said manually movable means beyond a' predetermined limiting range of its movement without similar movement of the other of said manually controllable means automatically to control said second source of bias potential to increase said current limit value in accordance with the difference oi. said motor excitation currents.

12. A motor control system for an electric vehicle comprising a direct current generator arranged to supply current to an output circuit, a plurality of direct current traction motors having their armatures connected in series circuit relation in said output circuit, separately excited individually controllable field excitation means for each of said motors, means for limiting the current in said output circuit to a selectable maximum value independently oi the effective resistance of said motor armatures, and control means operable upon reversal of the excitation of only one of said motors to increase said current limit value. r

13. A motor control system for an electric vehicle comprising a direct current generator arranged to supply current to an output circuit, a plurality of direct current traction motors connected to laterally spaced driving, elements of said vehicle and having their armatures connected in series circuit relation in said output circuit, separately excited individually controllable field excitation means for each of said motors, means for limiting the current in said output circuit to a maximum value selected in accordance with the speed of said vehicle independently of the effective resistance of said motor armatures, and control means operable upon reversal of the excitation of either of said motors without reversal of the excitation of the other of said motors to increase the current limit value at the instantaneous speed of operation.

14. A motor control system for a self-propelled electric vehicle comprising a direct current generator arranged to supply current to an output circuit, a plurality of direct current traction motors connected to laterally spaced driving wheels of said vehicle and having their armatures connected in series circuit relation in said output circuit, separately excited individually controllable field excitation means for each of said motors, means for limiting the current in said output circuit to a selectable maximum value independently of the effective resistance of said motor armatures, control means operable upon reversal of the excitation of either of said motors to increase the current limit value at the instantaneous speed of operation, and means operable in response to substantiall simultaneous reversal of the excitation of both said motors for rendering said control means ineffective to modify said current limit value.

15. A control system for an electrically driven vehicle comprising a direct current generator connected to supply current to an output circuit, a pair of direct current motors connected to laterally spaced driving wheels of said vehicle and having their armatures connected in series circuit relation in said output circuit, differential excitation means for controlling the excitation of said generator in accordance with the current in said output circuit, said differential excitation means comprising a current responsive element and a resistor connected in series c rcuit relation across a portion of said output circuit, means for limiting the current in said output circuit to a selectable maximum value comprising a unidirectionally conducting shunt circuit for said resistor including a source of bias potential operatively connected in said shunt circuit to oppose the voltage drop across said resistor, separate manually controllable means for supplying independently variable and reversible excitation currents to said motors, a pair of potentiometers connected in parallel circuit relation across said source of bias potential and each actuable by one of said manually controllable means to reduce the bias potential applied to said shunt circuit, means including switching means operable in response to movement of one of said manually controllable means to reverse the excitation of one of said motors selectable to render one oi said potentiometers eflective to increase said maximum current limit value, said switching means being operable upon like excitation reversing movement of both said manually controllable means to render said potentiometers ineflectlve to control said maximum current limit value.

GEORGE M. ADAMS. JOHN C. AYDEID'I'I. 

